New developments in the visual cycle: functional role of 11-cis retinyl esters in the retinal pigment epithelium

Although both 11-cis and all-tmns retinyl esters exist in the retinal pigment epithelium, the relative importance of each in the visual cycle has been unclear. Recent data indicate that there are 2 biochemical pathways leading to the formation of 11-cis retinoids from the retinal pigment epithelium pool of retinyl esters. One well-established pathway is located in the endoplasmic reticulum where all-trans retinyl esters are hydrolyzed, isomerized, and then oxidized to form 11-cis retinal (endoplasmic reticulum pathway). A more recently identified pathway resides within the plasma membrane where 11-cis retinyl esters are hydrolyzed directly to 11-cis retinol (plasma membrane pathway). Either or both pathways may provide 11-cis retinoids for regeneration of rod and cone visual pigments. Recent reports have suggested that the regeneration of rod and cone pigments are carried out by different mechanisms, and that 11-cis retinyl esters (plasma membrane pathway) in the retinal pigment epithelium may be specific to cone pigment regeneration. In this paper we review both visual pathways and consider data in support of the hypothesis that 1 of these 2 pathways' (the plasma membrane pathway) functions to provide visual pigment chromophores selectively for cone photoreceptors.